A global classification of hydrologic functional diversity in gauged and ungauged catchments

Abstract

Most of the world’s streams remain ungauged—a longstanding hydrologic challenge. Here we develop a globally scalable, seasonally resolved framework that classifies catchments according to the complexity of how rainfall is converted into runoff across events, yielding three functional types: simple, intermediate and complex. Applied to over 80,000 gauged and ungauged catchments, it reveals that functional complexity is the global norm: catchments draining 87% of the evaluated ungauged land area are classified as complex in the dormant season, with the number of complex catchments increasing by 63% in the growing season. Africa and much of Asia (except Japan) remain predominantly complex year round, Europe reaches 94% complex in the growing season, the US Pacific Northwest and Canada’s British Columbia retain simple clusters, and southeastern South America and southeastern Australia show notable transitions from complex to intermediate, demonstrating that catchment functional type is not fixed. Greater urban and agricultural land cover, together with less persistent rainfall, are associated with greater functional complexity. These findings provide a blueprint for prioritizing streamflow gauging in under-represented complex catchments and for guiding model structural selection, transferability and prediction in ungauged catchments, ensuring that models are neither simpler than necessary nor more complex than required. This study presents a globally scalable, seasonally resolved classification of gauged and ungauged catchments into three functional rainfall–runoff types (simple, intermediate and complex), revealing that functional complexity is the norm across most of the world’s rain-dominated streams and rivers and providing a framework to guide streamflow gauging network prioritization and hydrologic model structural design.